Polylactic acid is a promising biopolymer having a low thermostability. Were it possible to achieve better thermal properties, the possible applications would increase greatly.
In order to be able to produce PLLA with optimal thermal properties, (optically) very pure L-lactide (L-LA) is required. Currently, the most used method for producing L-lactide includes a two-stage polycondensation of lactic acid to form an oligomer followed by a depolymerization. Because of the prevailing high temperatures, which are required for a rapid reaction course, and also because of cationic impurities of the lactic acid or of the reaction vessels (e.g. by corrosion), racemisation can occur, as a result of which meso-lactide is produced as by-product. This product must be separated from the main product since meso-lactide (M-LA) has a negative effect on the properties of the polymer produced during the polymerization of L-lactide. The result thereby is a notable reduction in the melting temperature and also in the glass transition temperature as shown in Table 1 below, while the mechanical properties likewise change.
TABLE 1PLLAPRLAPMLA (a/s)sc-PLAsbc-PLATg ° C.55-6050-5540-45/3480-9050-55Tm ° C.140-170—   —/153210-230185-195Tg: glass transition temperatureTm: melting pointPLLA: L-polylactic acidPRLA: racemic polylactic acidPMLA: meso-polylactic acida: amorphouss: syndiotacticsc: stereocomplexsbc: stereoblock copolymer
Meso-lactide, like L-lactide, is a cyclic diester with two optically active carbon atoms in the ring. It has an optical R- and an S-center and is therefore optically inactive. The polymerization of meso-lactide leads to an amorphous polymer. A syndiotactic polymer can be produced using a stereoselective catalyst (Tina M. Quitt and Geoffrey W. Coates, J. Am. Chem. Soc. 1999, 121, 4072-4073), the thermal properties of which are however poorer than those of PLLA.
Stereocomplexes of polylactic acid (PLA) can resolve the problem of low thermal stability but the optical counterpart of L-polylactic acid (PLLA) is required for the production of stereocomplexes. D-polylactic acid (PDLA) is available only in small quantities and is very expensive.
Rac-lactide has been obtained to date from equal quantities of D,D- and L,L-lactide by melting. Since D,D-lactide is relatively expensive because of the great complexity for producing D-lactic acid, reuse as a monomer for the polylactic acid production has to date been more of theoretical interest. The properties of D,D-L,L-stereopolymers are thereby of great interest since they have significantly better thermostabilities and hence could eliminate one of the disadvantages of polylactic acid.
Dilactides that are composed of the enantiomers of lactic acid are already known. WO 1984/04311 A1 describes a method for the production of a polymer from caprolactone and lactide that is used for the production of everyday objects in medicine and care technology. The dilactide is commercially available and is predominantly composed of the two enantiomers of lactic acid, L-(−)- and D-(+)-lactic acid. This mixture is frequently associated with dilactide and includes the same enantiomers of lactic acid, namely D-lactic acid or L-lactic acid. No allusion to the production of these dilactides composed of the same enantiomers is given.
The polymerization of a mixture of meso-lactide and L-lactide leads to a copolymer, the thermal properties of which are inferior to those of PLLA. Meso-lactide can also be used in the production of racemic lactic acid (D/L-LA) by hydrolysis with water. However, these applications are only of subordinate interest from a commercial point of view so that an increase in the economic value is sought.